The instant invention relates to bimetallic disc-actuated thermostatic switches and more particularly to a bimetallic disc assembly therefor which is adapted for minimizing the response time of the switch.
It is known that many household, office and industrial machines and appliances now include highly sensitive electrical components that may be damaged by thermal overload conditions. In this regard, thermostatic switches have heretofore been known as high limit devices for preventing thermal overload damage to machines or appliances containing such sensitive electrical components. In this regard, thermostatic switches are operative for interrupting the electrical connections of the main heat generating components of the machine or appliance in response to thermal overload conditions. However, since it has been found that thermal overload conditions can develop very rapidly, it is essential for the thermostatic switches which are used in these devices to have a rapid response time in order to avoid damage to the machine components.
The U.S. patents to Craig, III U.S. Pat. No. 4,754,252 and Chrupcala U.S. Pat. No. 4,952,901, which represent the closest prior art to the subject invention of which the applicant is aware, are specifically concerned with minimizing response time in thermostatic switches. The Craig patent concerns a disc retainer and cap assembly which cooperate to increase air flow across the surface of the bimetallic disc. The cap has a plurality of air circulation vents extending therethrough and the disc support or retainer is designed so that exterior ambient air can pass through the air circulation openings and across the underside of the bimetallic disc to provide increased responsiveness to ambient conditions. While the Craig device is effective for reducing response time, it also presents several design problems which make it difficult to assemble. In particular, the radial disc supports must be radially aligned with the areas between the vents so that the disc supports do not block air flow through the vents. It can be appreciated that this rotational alignment is very difficult to maintain while the cap is positioned on the switch body and crimped in place.
The patent to Chrupcala discloses a disc retainer and aluminum cap assembly which is adapted to minimize the conduction of heat from the bimetallic disc to the disc retainer. The disc retainer includes a central hub portion, a plurality of fingers which radiate outwardly from the hub portion, a plurality of disc retainer projections on the fingers and a plurality of disc support shoulders on the fingers adjacent the disc retainer projections. The fingers have open notches therebetween for reducing the cross-sectional area of the disc retainer which may conduct heat from the bimetallic disc to the body portion of the thermostatic switch. The aluminum cap includes an end wall portion having an enlarged opening therein and a skirt portion having a plurality of apertures or slots therein which reduce the cross-sectional area of the skirt for conducting heat from the bimetallic disc to the body portion of the switch. The Chrupcala device also presents several problems. More specifically, the material from which the disc retainer can be formed is limited to plastic thermosetting materials which are easy to mold and resistant to breakage during manufacturing and handling. The problem here is that thermosetting materials have a relatively low melting point which limits use of the switch to applications where temperature does not exceed 375-400 degrees F. In addition, it has been found that it is difficult to crimp the aluminum perforated cap, and it has further been found that the crimping of the cap tends to deform or warp the cap, which may result in pressure on the disc which in turn may adversely affect the snap-action thereof. It is pointed out that these types of disc retainers cannot be formed from a ceramic material because the fingers would be very brittle and would very easily break during the manufacturing process.
The instant invention provides an improved bimetallic disc assembly for a bimetallic disc-actuated thermostatic switch. In this regard, the disc assembly includes an integrally molded ceramic disc retainer, a bimetallic disc which is received on the disc retainer, and a metallic cap which is received over the disc retainer. The disc retainer is adapted to minimize heat flow from the metallic cap to the disc retainer by minimizing the areas of contact between the disc retainer and the cap. The ceramic construction of the instant disc retainer also allows it to be used in a broader temperature range than the prior art plastic disc retainers. In particular, the instant disc retainer comprises a central hub having an axial bore extending therethrough, a circular flange extending radially outwardly from the central hub, a circular support shoulder extending upwardly from the flange, and a peripheral rim extending upwardly from the support shoulder. The peripheral rim has a plurality of circumferentially spaced projections, or bumps which extend upwardly therefrom. The metal cap is received on the disc retainer so that it rests on top of the projections or bumps on the peripheral rim. The bumps effectively minimize the contact area between the cap and the disc retainer and thereby reduce the transfer of heat from the cap to the disc retainer. By reducing the transfer of heat from the cap to the disc retainer, heat is more quickly transferred from the cap to the bimetallic disc, thereby improving response time.
Accordingly, it is an object of the instant invention to provide a thermostatic switch which has a quick response time to changes in wide ranges of ambient temperature conditions.
It is another object to provide a disc retainer for a thermostatic switch which may be made of ceramic and which is adapted to increase response time to changes in ambient temperature conditions.
It is still another object to provide a disc retainer which minimizes contact area between the cap and disc retainer.
It is still another object to provide a thermostatic switch which can be used in a broad range of temperatures.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.